Very strange mattresses.. should include in the description that it comes in 4 pierces.. 3 large pieces of foam into a zippered cover. I don’t really consider that a mattress but I guess for the price I shouldn’t have expect much. One of the pieces of foam was indented so I’m hoping it fills back out. 
I ordered two of these for my kids Bunkbeds which haven’t arrived yet so I can’t comment on the fit. 
Seems very firm.. hopefully my kids like them.. otherwise it was a waste of 300